Beverage bottle caps are made of two pieces. The hard outer piece is made from polypropylene. The inner liner is made from ethylene-vinyl acetate copolymer (EVA). Polypropylene is not used for the inner liner because processing polypropylene needs a high level of processing additives. The processing additives may leach out into the beverage contained in the beverage bottle and thus contaminate the beverage. EVA has excellent sealing ability and organoleptic properties, but it is expensive. Two-pieces caps, while satisfying the physical and organoleptic requirements for beverage bottles, are expensive and difficult to process because of the relatively high cost of EVA and the complication of the multi-piece fabrication.
Single-piece beverage bottle caps are known. For instance, U.S. Pat. Appl. Pub. No. 2008/0033111 discloses single-piece beverage bottle caps made from a polyethylene blend which comprise a first polyethylene and optionally a second polyethylene. The first polyethylene has a multimodal molecular weight distribution such that the ratio of weight average molecular weight of the polymer to the number average molecular weight of the polymer is at least about 12. Single-piece bottle caps are advantageous over two-piece bottle caps because they are simple to make and easy to use. The production rate of single-piece polyethylene caps is largely controlled by the crystallization rate of polyethylene.
Furthermore, most beverages, such as fruit juices, sweetened teas, carbonated beverages, and milk, are acidic. Polymers for making beverage bottle caps thus require acid resistance which can be measured by the environmental stress cracking resistance (ESCR). The polyethylene cap disclosed by U.S. Pat. Appl. Pub. No. 2008/0033111 has relatively low ESCR.
A new polyethylene composition is needed. Ideally, the polyethylene composition would have both improved ESCR and faster crystallization rate.